1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. Specifically, the invention relates to an air/fuel ratio control system in which engine torque is controlled such that the engine may run as lean as possible, particularly at a time between engine start and engine warming.
2. Description of the Related Art
Generally, in internal combustion engines, at a time between engine starting and engine warming when fuel combustion is carried out at relatively low temperature, a present air/fuel ratio is increased to bring the present air/fuel mixture toward a theoretically determined air/fuel ratio for promoting stable fuel combustion.
Thus, conventionally, an air/fuel ratio at engine starting is set to be on the `rich` side to allow for different operating environments, etc. However, if the air/fuel setting is rich beyond an amount appropriate to the present operating conditions, fuel combustion and engine exhaust characteristics are degraded and engine performance is reduced.
Recently, for improving combustion and fuel efficiency, so-called lean burning engines, utilizing an air/fuel ratio (e.g. 20-25) lower than a theoretical air/fuel ratio (i.e. 14.7) have been introduced. The present applicant has disclosed in Japanese Patent Application First Publication 4-963, an air/fuel ratio control system which is effective to detect and suppress surges in the air fuel ratio due to torque and speed variations in the engine within predetermined limits.
According to the above-cited disclosure, at the time of engine starting, a correction value for enrichment of the air/fuel ratio is kept as small as possible. Thus, at the time of engine starting, the air/fuel ratio is set to be lean to the limit of surging. According to this system however, a possibility that insufficient torque is generated is present which may cause engine hesitation or stumbling. In a worst case engine stalling may occur.
Accordingly, the present invention discloses an air/fuel ratio control system in which engine surge torque is detected such that the engine may run as lean as possible, particularly at a time between engine start and engine warming.